How do I learn to let it go?!

Hey guys, buckle up for a ride!
Some backstory- My daughter is now 7 years old.. we just celebrated her birthday this past Sunday. Bio father not in the picture due to addiction. My Ex mil is an absolute nightmare. Never met someone so narcissistic. Ex Mil has been an addict for 25+ years, sober on and off this whole time involving several duis and arrests. Abandoned daughters bio father several times throughout his early childhood and still into adulthood as her latest relapse was in September 2021. Due to her past when daughter was born my ex husband said he didn’t want daughter around her, I agreed. When he let addiction get the best of him after only meeting daughter 3 times in 6 years at the time, decided to reach out for a relationship. Was hesitant due to her past, but agreed. We met with her about 3 more times in 6 months or so before I heard about the relapse, in September as I mentioned. I cut contact immediately with no explanation

Fast forward to Monday, I find out she’s using pictures of my daughter and posting them all over Facebook playing house and writing posts about my daughters birthday. And when I confronted her, saying please don’t post my daughter on social media and play house with her, she tried to go toe to toe with me and saying I’m wrong and she’ll always be a GrAnDmOthEr, not an I’m sorry won’t happen again but then tried to gaslight me and said it’s my fault she doesn’t have a relationship with my daughter because I don’t allow it. WTF? Like yes I don’t allow it but why don’t I allow it? I explained very thoroughly I don’t want my daughter around addicts. But makes it my fault that I don’t allow them to have a relationship.

I’m at my wits end with this stupidity and trying to comprehend it, I try to do the right things and I feel like my mental health deteriorates listening to this hogwash. Any advice on how to learn to move on and accept she’s just going to do and say whatever she wants?

How do I learn to let it go?!